This invention relates generally to the art of frequency synthesization, and more particularly, to the art of utilizing phase locked loops for frequency synthesis.
Discrete frequency generation and synthesis have long been important in the electrical arts. As electronic technology has become more sophisticated, frequency synthesizers, which "build" desired frequencies in several steps, have also become sophisticated and complex, particularly where frequency stability and resolution are important. In general, there is a continuing need for frequency synthesizers having reduced noise characteristics, improved resolution, particularly at the lower frequencies, and increased sensitivity, all to be achieved at minimum cost. The phenomenon of spurious signals and other noise in frequency snythesizers is very important and is a significant drawback of the prior art. The introduction of noise into a synthesizer will vary in its effect, depending upon the point at which the noise occurs. At the output, however, the noise will degrade performance, set effective power limits, and will require specialized circuitry to minimize its effects. In addition to the problem of noise, current synthesizer techniques often require internal frequencies of 6 to 10 times the highest output frequency, thereby increasing the complexity and cost of the synthesizer to achieve adequate resolution and control.
The prior art frequently makes use of mixer devices to achieve a specific range of output frequencies. The use of mixers in the final stages require high (as compared to the output) internal frequencies and frequently is a source of noise, thereby reducing resolution of the entire system.
Furthermore, the prior art which utilizes phase locked loops, such as shown by the patent to Muraszko, U.S. Pat. No. 3,202,930, and the patent to Hoo, U.S. Pat. No. 3,379,992 have disadvantages because of range and resolution limitations. Both of these patents, although eliminating the output mixer, are restricted in their range and the control over the output. Utilizing the techniques disclosed in these patents to cover a frequency range of close to 100 MHz, with a 1 Hz resolution would require a great number of phase locked loop circuits and additional control circuitry.
In view of the background of the invention described above, an object of the present invention is to provide a frequency synthesizer utilizing phase locked loops having a wide range of output frequencies.
It is another object of the present invention to provide a frequency synthesizer having a high resolution of output frequencies.
It is a further object of the present invention to provide a frequency synthesizer in which the highest output frequency is not significantly less than the highest frequency produced with the synthesizer.
It is a still further object of the present invention to provide a frequency synthesizer having a plurality of individually controlled frequency circuits, each circuit being relatively independent of each other.
Other and futher objects, features and advantages of the present invention will become apparent as the description proceeds.